With an ever growing need for higher data communication rate over wireless medium, users are demanding high rate data transmission. One area in which the aforementioned considerations have arisen is in UMTS Terrestrial Radio Access Network (UTRAN) and Evolved-UTRA, which call for higher user data rates and improved quality of service. A number of proposals have discussed and concluded the need for multiple-antenna systems to achieve the target spectral efficiency, throughput, and reliability of EUTRA. These proposals have considered different modes of operation applicable to different scenarios. The basic assumptions that vary among proposals include (i) using single stream versus multiple streams, (ii) scheduling one user at a time versus multiple users, (iii) having multiple streams per user versus a single stream per user, and (iv) coding across multiple streams versus using independent streams. A common factor among various downlink physical layer multiple-input-multiple-output (MIMO) proposals, however, is a feedback strategy to control the transmission rate and possibly vary the transmission strategy.
MIMO antenna systems can be effective in fading environment by providing significant performance improvements and achievable data rates in comparison to single antenna systems. Closed loop MIMO systems typically transmit channel state information from a receiver to a transmitter. Transmitting the channel state information consumes bandwidth that might otherwise be available for data traffic. The performance gain achieved by multiple antenna system increases when the knowledge of the channel state information (CSI) at each end, either the receiver or transmitter, is increased. Although perfect CSI is desirable, practical systems are usually built only on estimating the CSI at the receiver, and possibly feeding back the CSI to the transmitter through a feedback link with a very limited capacity. Using CSI at the transmitter, the transmission strategy is adapted over space (multiple antennas) and over time (over multiple blocks).
Conventional DL MIMO transmission strategy includes quantized precoding schemes, antenna selection, antenna cycling (with or without rank control), and different space-time coding and spatial multiplexing transmission scheme. The quantized precoding schemes are used whenever a feedback link from the receiver to the transmitter is present. The performance of quantized precoding scheme depends on the codebook design and the complexity and memory requirement are further functions of the precoding algorithm. Hence, codebook design is one of the main aspects of the quantized precoding schemes. The conventional design criterion is based on SNR maximization which is not necessarily the capacity-optimal design strategy. Moreover, the precoder size is assumed to be fixed which means the number of transmitted data streams, i.e., precoding rank, is fixed and is usually taken to be the minimum of the number of transmit and receive antennas.